Under present conditions, petroleum refineries are finding it necessary to convert increasingly greater proportions of crude to premium fuels such as gasoline and middle distillates such as diesel and jet fuel. Due to recent environmental legislation, "clean fuels" or fuels which are low in sulfur, content combust without leaving carbon residue, and have reduced harmful emissions, are in increasing demand. Sulfur contributes to engine wear and the corrosion of mufflers and exhaust pipes. Diesel fuels, according to conventional product specifications, have a cetane number of at least 45. Cetane number is a measurement of the ignition quality of diesel fuel and is directly related to cleanliness of fuel. Diesel may be contaminated by even small amounts of dirt. A relatively high cetane number is indicative of a fuel which enables an engine to operate smoothly and easily at low temperatures. It has been discovered that diesel fuels of high cetane number, 55 or greater at less than or equal to 41.degree. F. (5.degree. C.) cloud-point, may be produced by hydrocracking light cycle oils from the fluid catalytic cracking process at high pressure, then catalytically dewaxing certain fractions from the hydrocracking process under low pressure.
Light cycle oil (LCO) is a highly aromatic, hydrogen-deficient middle distillate having high levels of sulfur and nitrogen. Table 1 below gives typical sulfur and nitrogen contents for a light cycle oil.
TABLE 1 ______________________________________ Light Cycle Oil Aromatics, pct. S, pct. N, ppm H, pct. ______________________________________ 57 1.6 700 11.57 ______________________________________
Because of its high content of aromatics and heteroatoms, LCO has been difficult to dispose of as a commercially valuable product. Formerly, the light and heavy cycle oils could be upgraded and sold as light or heavy fuel oil, such as No. 2 fuel oil or No. 6 fuel oil. Upgrading the light cycle oil was conventionally carried out by a relatively low severity, low pressure catalytic hydro-desulfurization (CHD) unit in which the cycle stock would be admixed with virgin mid-distillates from the same crude blend fed to the catalytic cracker. Further discussion of this technology is provided in the Oil and Gas Journal, May 31, 1982, pp. 87-94.
Currently, however, the refiner is finding a diminished demand for fuel oil. At the same time, the impact of changes in supply and demand for petroleum has resulted in a lowering of the quality of the crudes available to the refiner; this has resulted in the formation of an even greater quantity of refractory cycle stocks. As a result, the refiner is left in the position of producing increased amounts of poor quality cycle streams from the catalytic cracker while having a diminishing market in which to dispose of these streams.
At many petroleum refineries, the light cycle oil (LCO) from the FCC unit is a significant component of the feed to the catalytic hydrodesulfurization (CHD) unit which produces No. 2 fuel oil or diesel fuel. The remaining component is generally virgin kerosene taken directly from the crude distillation unit. The highly aromatic nature of LCO, particularly when the FCC unit is operated in the maximum gasoline mode, increases operational difficulties for the CHD and can result in a product having marginal properties for No. 2 fuel oil or diesel oil, as measured by cetane numbers and sulfur content.
In the past there have been difficulties in employing LCO in the preparation of diesel fuel. Diesel fuel must meet a minimum cetane number specification of about 45 in order to operate properly in typical automotive diesel engines. Because cetane number correlates closely and inversely with aromatic content, the highly aromatic cycle oils from the cracker typically with aromatic contents of 80% or even higher have cetane numbers as low as 4 or 5. In order to raise the cetane number of these cycle stocks to a satisfactory level by the conventional CHD technology described above, substantial and uneconomic quantities of hydrogen and high pressure processing would be required.
Because of these problems associated with its use as a fuel, recycle of untreated light cycle oil to the FCCU has been proposed as a method for reducing the amount of LCO. Benefits expected from the recycle of LCO include conversion of LCO to gasoline, backout of kerosene from No. 2 fuel oil and diminished use of cetane improvers in diesel fuel. However, in most cases, these advantages are outweighed by disadvantages, which include increased coke make in the FCC unit, diminished quality of the resultant LCO and an increase in heavy cycle oil and gas.
A typical LCO is such a refractory stock and of poor quality relative to a fresh FCC feed that most refineries do not practice recycle of the untreated LCO to any significant extent. One commonly practiced alternative method for upgrading the LCO is to hydrotreat severely prior to recycle to the catalytic cracker or, alternatively, to hydrotreat severely and feed to a high pressure fuels hydrocracker. In both such cases, the object of hydrotreating is to reduce the heteroatom content to low levels while saturating polyaromatics to increase crackability. In those instances where the production of gasoline is desired, the naphtha may require reforming to recover its aromatic character and meet octane specifications.
Hydrocracking may be used to upgrade the higher-boiling more refractory products derived from catalytic cracking. The catalytic cracker is used to convert the more easily cracked paraffinic gas oils from the distillation unit while the hydrocracker accepts the dealkylated, aromatic cycle oils from the cracker and hydrogenates and converts them to lighter oils. See Petroleum Refining; Second Ed.; Gary, J. H. and Handwerk, G. E.; Marcel Dekker, New York 1984; pp. 138-151; Modern Petroleum Technology, Fourth Ed.; Hobson, G. D., Applied Science Publ. 1973; pp. 309-327. A notable advance in the utilization of FCC cycle oils is described in U.S. Pat. No. 4,676,887. It was found that highly aromatic, refractory feeds derived from catalytic cracking could be converted directly to high octane gasoline by hydrocracking at relatively low pressures, typically 600-1000 psig (about 4250-7000 kPa. abs.) and with low conversions, typically below 50 weight percent to 385.degree. F. (195.degree. C.) products. By using a highly aromatic feed which has been substantially dealkylated in the catalytic cracking operation, typically with an API gravity of 5-25, the hydrocracking proceeds with only a limited degree of aromatics saturation so that a large quantity of single-ring alkylaromatics (mainly benzene, toluene, xylenes and trimethyl benzenes) are obtained by ring opening of partial hydrogenation products of bicyclic aromatics. The single ring aromatics are not only in the gasoline boiling range but also possess high octane numbers so that a high octane gasoline is produced directly, suitable for blending into the refinery gasoline pool without prior reforming.
Hydrocracking is, however, not well adapted to the production of high cetane diesel, as opposed to gasoline because the hydrocracked product contains significant amounts of iso-paraffins produced by isomerization and ring opening reactions characteristic of hydrocracking. While iso-paraffins are conducive to high octane ratings in gasoline they tend to lower the cetane number of diesel fuels (See Modern Petroleum Refining, Hobson). These difficulties apply with particular force in high pressure hydrocracking in which these reactions are favored. In addition, diesel fuel from hydrocracking, particularly in the higher boiling fractions, tends to have poor cold flow properties (e.g. pour, cloud, freeze points) due to the large amount of long chain paraffinic components. These properties deleteriously affect diesel fuel potential by limiting distillate yields and producing fuel outside of conventional specifications. For these reasons, high pressure hydrocracking has not been considered as an appropriate process for the production of high cetane number diesel fuel. Hydrocracker processes of this type are described, for example, in U.S. Pat. No. 3,306,839, issued to Vaell, which discloses a gas oil feed passing through a hydrotreater. It is then cascaded to a first stage hydrocracker. The hydrocracked effluent passes to a first stage high pressure separator and then to a common low pressure separator. The first stage hydrogen circuit is provided with recycle hydrogen from the first stage high pressure separator. The stream from the low pressure separator goes to the fractionator with the bottoms fraction being passed to the second stage hydrocracker together with recycle hydrogen from the second stage high pressure separator. The second stage high pressure separator is connected to the common low pressure separator so that the combined hydrocracked products are sent to the fractionator. U.S. Pat. No. 3,256,177, issued to Tulleners,discloses an arrangement similar to that of Vaell. A gas oil feed passes to a high pressure hydrotreater with its effluent cascaded to a first stage hydrocracker. First stage hydrocracker effluent passes to the high pressure separator and low pressure separator and then to the fractionator. Under one process alternative the fractionator bottoms passes to a second stage hydrocracker after mixing with fresh hydrogen and preheating. Separate second stage high pressure separators and low pressure separators are provided and the condensate from the low pressure separator is returned to the fractionator.
U.S. Pat. No. 3,174,925, issued to Claussen, also discloses two stage hydrocracking with bottoms feed to the second stage.
U.S. Pat. No. 4,985,134, issued to Derr, et al. is directed to the production of both gasoline and middle distillate fractions. Primarily gasoline boiling range products are to be produced in Derr, et al., since claim 1 recites conversion to such products of no more than 75%. Derr et al, furthermore operates at low pressure, since claim 1 of Derr, et al. specifically states that the hydrogen partial pressure is to be below 1200 psig. The instant invention, on the other hand, is directed to high pressure operation.
Gasoline may be produced in the hydrocracking step of the instant invention, but only as a by-product. Derr et al is directed to the deliberate production of both gasoline and diesel products.
Applicants seek to maximize the production of high cetane diesel fuel. Derr et al. actually teaches away from the instant application. Table 2 of Derr et al. indicates that the cetane index of the distillate fraction produced in Derr et al (between 35 and 38) is far below the cetane number of the instant invention. The instant invention employs a cetane number of at least 55 at less than or equal to 5.degree. C. cloud point. Derr et al. does not discuss cloud point. Cetane number is an experimentally measured characteristic, using ASTM engine test D 613. Cetane Index is a calculated value which is known in the art of diesel production. Cetane Index approximates Cetane Number, using API gravity and mid-boiling point. The results are quite close to those obtained by direct experimental measurement. Cetane number and cetane index are therefore comparable. Derr et al states (col. 4, lines 47-50) that the distillate of its invention has a very low cetane blending value, making it unacceptable for use as a road diesel fuel. This is not true for the distillate of the instant invention.
Furthermore, there is no teaching in Derr, et al of catalytic dewaxing of the hydrocracker effluent. The primary product of Derr, et al. is gasoline, and the presence of substantial quantities of wax in a light, volatile fraction such as gasoline is unlikely.
The instant invention comprises a high pressure hydrocracking process in which the primary product is high quality diesel fuel. Derr et al. is a low pressure hydrocracking process directed to the production of gasoline and distillate products which are not road quality diesel.
U.S. Pat. No. 4,483,760, issued to Tabak et al, is directed to the production of middle distillates and fuel oils, which are heavier than diesel fuels. Tabak et al indicates (col. 2, lines 26-28) that its catalytic dewaxing process may be used to lower unacceptably high pour points. The instant invention is not directed to the lowering of pour points or cloud points. The instant invention seeks to maximize cetane number. FIG. 2 of the instant application illustrates a direct relationship between cloud point and cetane number. The higher the cloud point, the higher the cetane number. The instant invention permits cloud points as high as 5.degree. C., which is 41.degree. F., so that cetane number can be as high as possible. Tabak et al does not discuss cetane number because it is a diesel characteristic and Tabak et al is directed to fuel oils. Tabak et al. is so concerned with pour point that it comprises contacting a dewaxing by-product with a dewaxing catalyst a second time in order to obtain an increased yield of dewaxed fuel oil.